LIGA technology (Lithographie Galvanik Abformung), developed by W. Ehrfeld of the Karlsruhe Nuclear Research Centre, Germany in the 1980s, has proved advantageous for fabricating high precision metallic microstructures.
The principle of LIGA technology consists in depositing, on a conductive substrate or substrate coated with a conductive layer, a photosensitive resin layer, in performing X ray radiation through a mask that matches the contour of the desired microstructure using a synchrotron, developing i.e. removing the non-radiated portions of the photosensitive resin layer by physical or chemical means, to define a mould that has the contour of the microstructure, in galvanically depositing a metal, typically nickel, in the photosensitive resin mould, then removing the mould to release the microstructure.
The quality of the microstructures obtained is not open to criticism, but the requirement to implement expensive equipment (the synchrotron) makes this technique incompatible with the mass production of microstructures that have to have a low unitary cost.
This is why, on the basis of this LIGA method, similar methods have been developed, but which use UV photosensitive resins. A method of this type is disclosed, for example, in the publication by A. B. Frazier et al., entitled “Metallic Microstructures Fabricated Using Photosensitive Polyimide Electroplating Molds”, Journal of Microelectromechanical Systems, Vol. 2, N deg. 2 Jun. 1993, for fabricating metallic structures by electroplating metal in polyimide based photosensitive moulds. This method includes the following steps:                creating a sacrificial metallic layer and a conductive priming layer for a subsequent electrodeposition step;        applying a photosensitive polyimide layer;        UV irradiating the polyimide layer through a mask that matches the contour of the desired microstructure;        developing, by dissolving, the non-irradiated parts of the polyimide layer so as to obtain a polyimide mould;        galvanically depositing nickel in the open part of the mould up to the height of the mould, and        removing the sacrificial layer and separating the metallic structure obtained from the substrate, and        removing the polyimide mould.        
The microstructures obtained in accordance with the methods of the prior art are metallic microstructures made of a single metal, generally nickel, and copper, nickel-phosphorus, which is not always optimal depending upon the application for which they are intended. Indeed, applications exist for which one or other of these materials does not have optimal properties, both from the mechanical and tribological point of view. Typically, a toothed wheel has to be sufficiently rigid to resist breakage if subjected to a high level of stress, but also has to have teeth with a low friction coefficient to facilitate gearing. The choice of nickel is thus very advantageous from the point of view of mechanical resistance, however nickel has less advantageous tribological properties, since it has a relatively high friction coefficient.
One way of solving this problem consists in making the core of the desired microstructure by a LIGA-UV method with a first metal, then coating the core with a layer of a second metal by another, conventional method, for example, by vacuum vapour deposition. This type of method has, however, the drawback of not allowing parts to be obtained simply with controlled geometrical precision. There therefore exists a need for a method that can overcome this drawback.